pool: avoid throwing a RuntimeException for non-bugs

Motivation:

The pool uses a lambda function to create a mover, wrapping any
CacheException thrown while creating the mover with a RuntimeException.

The code that handles RuntimeExceptions is faulty, as it checks whether
the root cause is a CacheException, rather than the immediate cause.
The CacheException may, itself, have a cause that isn't a
CacheException.

The result is that a CacheException is re-thrown as a RuntimeException,
which is then logged as a bug in dCache.

Modification:

Use a custom RuntimeException (UncheckedCacheException) to wrap the
CacheException within the lambda.  The catch block is now specific
enough to catch only the CacheException, leaving other RuntimeExceptions
to propagate.

Result:

Certain error cases, where a pool is unable to create a mover are no
longer logged as a bug in dCache.

Target: master
Request: 5.1
Request: 5.0
Request: 4.2
Request: 4.1
Request: 4.0
Request: 3.2
Requires-notes: yes
Requires-book: no
Patch: https://rb.dcache.org/r/11688/
Acked-by: Tigran Mkrtchyan

modules/dcache/src/main/java/org/dcache/pool/classic/MoverRequestScheduler.java

@@ -1,8 +1,8 @@
 package org.dcache.pool.classic;
 
-import com.google.common.base.Throwables;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+
 import java.io.InterruptedIOException;
 import java.nio.channels.CompletionHandler;
 import java.util.ArrayList;
@@ -54,6 +54,22 @@ public class MoverRequestScheduler
     private static final long DEFAULT_LAST_ACCESSED = 0;
     private static final long DEFAULT_TOTAL = 0;
 
+    /**
+     * A RuntimeException that wraps a CacheException.
+     */
+    private static class UncheckedCacheException extends RuntimeException
+    {
+        private UncheckedCacheException(CacheException cause)
+        {
+            super(cause.getMessage(), cause);
+        }
+
+        private CacheException getCacheException()
+        {
+            return (CacheException) getCause();
+        }
+    }
+
     /**
      * The name of IoScheduler.
      */
@@ -199,7 +215,7 @@ public int getOrCreateMover(MoverSupplier moverSupplier, String doorUniqueId,
                                                          try {
                                                              return createRequest(moverSupplier, key, priority);
                                                          } catch (CacheException e) {
-                                                             throw new RuntimeException(e);
+                                                             throw new UncheckedCacheException(e);
                                                          }
                                                      });
 
@@ -217,10 +233,8 @@ public int getOrCreateMover(MoverSupplier moverSupplier, String doorUniqueId,
             }
 
             return request.getId();
-        } catch (RuntimeException e) {
-            Throwable t = Throwables.getRootCause(e);
-            Throwables.throwIfInstanceOf(t, CacheException.class);
-            throw e;
+        } catch (UncheckedCacheException e) {
+            throw e.getCacheException();
         }
     }